1. Technical Field
The present invention relates to a semiconductor device including a thin-film transistor including a polycrystalline silicon layer, disposed above a substrate, serving as an active layer; a method for manufacturing the semiconductor device; and an electro-optical apparatus.
2. Related Art
Thin-film transistors are used to control currents applied to organic electroluminescent elements disposed in electro-optical apparatuses or are used in liquid crystal apparatuses containing analogue circuits, disposed on substrates, including as operational amplifiers. The thin-film transistors are used for these applications because of their saturation characteristics. The saturation characteristics of the thin-film transistors are less complete than those of MOS transistors formed on silicon substrates. Therefore, the following phenomenon occurs in the thin-film transistors: a phenomenon that drain currents increase due to variations in drain voltages. Phenomena similar to such a phenomenon are described below with reference to FIG. 10.
First Phenomenon
As shown in FIG. 10, in a thin-film transistor, the drain current increases in the high drain voltage region because of a phenomenon called a kink effect. This increases the ratio of a change in the drain current to a change in the drain voltage. Such a phenomenon is probably caused as described below. In the thin-film transistor, if the drain voltage increases to exceed the pinch-off voltage, a relatively large electric field is concentrated on an end of the drain. When the electric field exceeds a certain intensity, electrons accelerated by the electric field cause impact ionization, thereby generating electron-hole pairs. Holes generated in a bulk MOS transistor by impact ionization flow toward a semiconductor substrate and therefore have no significant influence on the source-drain current. Holes generated in the thin-film transistor by impact ionization flow into the channel zone to reduce the potential of the channel zone because the channel zone has no contact with these holes, thereby increasing the electron current. In order to prevent the electric field from being concentrated on the drain end, the following structure may be used: a lightly doped drain (LDD) structure in which a semiconductor layer has a lightly doped region opposed to an end portion of a gate electrode. However, the LDD structure is insufficient to completely suppress the kink effect.
Second Phenomenon
For an enhancement transistor, particularly a bulk MOS transistor, a operating point in which the drain voltage that is Vth less than the voltage Vds=Vgs corresponds to the pinch-off voltage Vp and the range of the source-drain voltage Vds corresponds to a saturation region. However, the pinch-off voltage of the thin-film transistor is unclear as shown in FIG. 10. That is, the linear region and saturation region of the thin-film transistor are separated from each other with a relatively wide voltage range. This is probably because the potential of the channel zone of the thin-film transistor depends on relations among the gate voltage, the drain voltage, and the source voltage. That is, the drain voltage probably influences the gate electrode through not only the semiconductor layer, through which a current flows, but also an insulator located on the side opposite to the gate electrode. If the LDD structure is used to cope with the first phenomenon, an LDD region usually acts as a parasitic resistor; hence, the effective drain voltage applied to the channel zone is small.
Third Phenomenon
In the thin-film transistor, a region between the region in which the source-drain current increases as described in the first phenomenon and the pinch-off voltage described in the second phenomenon is referred to as a saturation region. As shown in FIG. 10, in the saturation region, the ratio of a change in the drain current to a change in the drain voltage is not sufficiently small. Therefore, there is a problem in that constant-current operation cannot be assured.
Design techniques for solving such a problem may use structures below.
Structure A
A thin-film transistor with a large channel length is effective in improving the third phenomenon. The increase in the channel length thereof reduces the intensity of an electric field acting in the drain direction and therefore improves the second phenomenon. However, in order to achieve sufficient characteristics, the channel length needs to be very large. The increase in the channel length increases the gate capacitance and therefore impairs high-frequency characteristics of a circuit. The increase in the channel length reduces the sensitivity to varying the gate current by varying the gate voltage. Furthermore, the increase in the channel length increases the area occupied by the thin-film transistor and therefore is limited.
Structure B
It is known that an LDD region that is formed at an end of the drain of a thin-film transistor such that the intensity of an electric field acting on the drain end is reduced. The first phenomenon can be improved in such a manner that the impurity concentration of the LDD region is set to be sufficiently small and the length thereof is set to be sufficiently large. However, the LDD region usually acts as a parasitic resistor and therefore limits the on-current of the thin-film transistor. The presence of the LDD region reduces the effective drain voltage and therefore causes the second phenomenon to be serious.
Structure C
FIG. 11A shows Structure C including a drain-side thin-film transistor TFTd and a source-side thin-film transistor TFTs connected to each other in series. A constant voltage Vbias is applied to the gate of the drain-side thin-film transistor TFTd. FIG. 11B shows current-voltage characteristics of the drain-side and source-side thin-film transistors TFTd and TFTs using the node voltage Vm as a parameter. With reference to FIG. 11B, broken lines show current-voltage characteristic curves of the drain-side thin-film transistors TFTd that have been obtained by varying the drain voltage Vd in this order: Vd1, Vd2, Vd3, and Vd4. Nodes of the current-voltage characteristic curve of the source-side thin-film transistor TFTs and the current-voltage characteristic curves of the drain-side thin-film transistors TFTd correspond to the operating currents of the drain-side and source-side thin-film transistors TFTd and TFTs connected in series. As shown in FIG. 11C, the saturation operation of Structure C is greatly improved. This connection is referred to as a cascode connection and is common among MOS analogue circuits. Structure C has a problem in that a circuit for generating the voltage Vbias applied to the gate of the drain-side thin-film transistor TFTd is necessary and a problem in the input range of the voltage Vgate applied to the gate of the source-side thin-film transistor TFTs is limited.
Structure D
FIG. 12A shows Structure D including a first thin-film transistor TFTd and second thin-film transistor TFTs connected to each other in series. The gates of the first and second thin-film transistors TFTd and TFTs are electrically connected to each other; hence, a voltage Vgate is commonly applied to the gates of the first and second thin-film transistors TFTd and TFTs instead of voltages Vbias and Vgate. This allows Structure D to operate as well as Structure C. FIG. 12B shows current-voltage characteristics of the first and second thin-film transistors TFTd and TFTs using the node voltage Vm as a parameter. With reference to FIG. 11B, broken lines show current-voltage characteristic curves of the first thin-film transistors TFTd that have been obtained by varying the drain voltage Vd. Nodes of the current-voltage characteristic curve of the second thin-film transistor TFTs and the current-voltage characteristic curves of the first thin-film transistors TFTd correspond to the operating currents of the first and second thin-film transistors TFTd and TFTs connected in series. As shove in FIG. 11C, the saturation operation of Structure D is greatly improved. Structure D is disclosed in the following documents: L. Mariucci et al, AM-LCD 2003, pp 57-60 (hereinafter referred to as Non-patent Document 1) and Woo-Jin Nam et al, IDW 2004, pp 307-310 (hereinafter referred to as Non-patent Document 2).
Japanese Unexamined Patent Application Publication No. 2004-361424 (hereinafter referred to as Patent Document 1) discloses a structure including a drain-side thin-film transistor TFTd and a source-side thin-film transistor TFTs connected to each other in series. The gates of the drain-side and source-side thin-film transistors TFTd and TFTs are electrically connected to each other. The quotient Wd/Ld obtained by dividing the channel width by the channel length of the drain-side thin-film transistor TFTd is greater than the quotient Ws/Ls obtained by dividing the channel width by the channel length of the source-side thin-film transistor TFTs. Furthermore, when the drain-side and source-side thin-film transistor TFTd and TFTs are both a n-type, the threshold voltage of the source-side thin-film transistor TFTs is set to be less than that of the drain-side thin-film transistor TFTd. These are effective in minimizing variations in the drain-side and source-side thin-film transistor TFTd and TFTs.
It is apparent that the operating point of the first thin-film transistor TFTd of structure D shown in FIG. 12A is limited to the vicinity of the pinch-off voltage Vp of the second thin-film transistor TFTs. When the operating point of the first thin-film transistor TFTd is in the linear operation region of the second thin-film transistor TFTs, no advantage can be achieved. In order to achieve a good operating point, the ratio of the quotient Wd/Ld to the quotient Ws/Ls is limited. When the ratio (Wd/Ld)/(Ws/Ls) is greater than four, the first phenomenon can be solved.
In the thin-film transistors, the ratio of a change in the source-drain current Id to a change in the source-drain voltage Vds is large in the vicinity of the pinch-off voltage Vp. Therefore, in order to solve the second phenomenon, the ratio (Wd/Ld)/(Ws/Ls) needs to be greatly increased. If layout is made in an ordinary design range, high-frequency properties of circuits are impaired because of an increase in gate capacitance and the area occupied by the thin-film transistors is increased.
In the structure disclosed in Patent Document 1 when the drain-side and source-side thin-film transistor TFTd and TFTs are of a n-type, the threshold voltage of the source-side thin-film transistor TFTd and TFTs is set to be less than that of the drain-side thin-film transistor TFTd, because the drain-side and source-side thin-film transistor TFTd and TFTs have different purposes. Therefore, there is a problem in that a operating point is present in a region where the ratio of Ids to Vds is large in the vicinity of the pinch-off voltage Vp of each thin-film transistor.